In U.S. patent application Ser. No. 11/979,760, filed 8 Nov. 2008, some of the inventors of the present application and their co-workers disclose a method for forming a metal pattern on a substrate via printing and electroless plating, which utilizes an ink composition comprising catalytic metals to print a pattern on a substrate, and then forms a metal film on the printed pattern via electroless plating. The ink composition used in this prior art method comprises a sulfate terminated (SO4−) polymer having a hydrophilic functional group, wherein the polymer can be used as a reducing agent and a dispersing agent simultaneously, so that catalytic metal ions may be reduced to atoms and homogenously dispersed in the ink composition before and after printing. Details of the disclosure in this US patent application are incorporated herein by reference.
The ink composition used in the aforesaid U.S. patent application Ser. No. 11/979,760 contains a water swellable resin as a binder enhancing the binding strength between the metal layer and the substrate. A suitable binder is a polymer having a hydrophilic moiety and a hydrophobic moiety, such as is poly(tetrafluoroethylene) sulfonate or polystyrene sulfonate. The inventors of the present application have been working on an ink composition for use in a method for forming a metal pattern on a substrate, which is free from a binder or a polymer having a hydrophilic functional group.
There are two types of plated-through-hole (abbreviated as PTH) technologies at the present, which are (a) electroplating with a conductive layer containing carbon black, graphite, conductive polymer or a mixture thereof formed on the through holes; and (b) electroless plating with a catalyst layer containing a catalytic metal such as Pd adsorbed on the through holes. The type (a) PTH technology requires an addition of a dispersing agent for the carbon black or graphite to be well dispersed in a solvent, or an addition of a binder agent which is thermally decomposable to enhance the adsorption of the carbon black or graphite to the substrate. The type (a) PTH technology also suffers drawbacks such as a low metal deposition ratio during the electroplating and non-uniform metal layers formed on the walls of the through holes.
Taiwan patent number I276702, issued 21 Mar. 2007, discloses an electroless plating process using a palladium nanocolloidal hydrophobic dispersion in an organic solvent such as n-hexane or cyclohexane as an activator for the electroless plating or related metal catalyzed reactions. The palladium nanocolloidal dispersion is prepared by reacting an ionic surfactant dissolve in water as a particle stabilizer, reducing agent and phase transfer emulsifier with palladium acetate under refluxing at 90-130° C. for 6-24 hours, recovering the upper layer of the reaction mixture containing palladium nanocolloids, and diluting the recovered upper layer in a low polarity organic solvent such as n-hexane or cyclohexane. The ionic surfactant used are those containing sulphate group (SO42−) or sulfonic group (SO3−). The palladium nanocolloidal dispersion prepared in this prior art involves a long and high temperature reaction, and an organic solvent, which are not environment friendly, and thus there are still rooms for improvements.
Poly(N-isopropylacrylamide) (abbreviated as PNIPAAm) is well known to be a water-soluble and hydrophilic polymer, that exhibits an extended chain conformation below the lower critical solution temperature (LCST) when it is in aqueous solution. PNIPAAm can also undergo a phase transition to an insoluble and hydrophobic aggregate above its LCST. The LCST of PNIPAAm is between 28-35° C. Randomly copolymerizing a small proportion of a different co-monomer in PNIPAAm copolymers raises the LCST. This is because the copolymerized co-monomer reduces the aggregation ability of the isopropyl group in the e PNIPAAm.